Recently, in accordance with the rapid progress in the data communication techniques, the demands for ultra-high speed computers, ultra-high frequency, and optical communications have been steadily increasing. However, there is a limit in meeting such demands by means of the conventional silicon devices, and therefore, there have been brisk research activities on compound semiconductor devices made of materials having superior properties. Of the materials for such compound semiconductor devices, GaAs has a good electron mobility of about 8500 cm.sup.2 /V.S and is superior in its own noise characteristics, so that it can be used on high speed devices such as MESFET (Metal Semiconductor FET) and HEMT (High Electron Mobility Transistor). Further, GaAs has an energy band gap of about 1.43 eV at the normal temperature, and therefore, the wave length of light is about 8800 .ANG. which is a wave length near infrared. Moreover, it has direct transition characteristics. Therefore, it can be used on laser diodes (to be called LD below) and on photo-diodes (to be called PD below).
The LD inductively releases light which is produced by the reunions of the holes and electrons which are injected from the PN junction of the semiconductor devices, and the released light has interferences and orientation characteristics. Meanwhile, the PD has the same structure as the LD, and an electric current flows through it when a reverse bias is applied and light rays are irradiated on it. Therefore, the LD and the PD are respectively used for transmitting and receiving in communication. Further, if the LD is used as a transmitting device, the intensity of the light rays released therefrom is made constant by using a monitoring PD (to be called MPD below). That is, the MPD detects the intensity of the light rays released from the LD, and controls the voltage which is supplied through an external circuit to the LD.
The LD and the MPD are combined by means of wires and bondings in the form of a hybrid, but their manufacturing cost is high, and there is a difference between the characteristics of the manufacturing processes of the LD and the MPD. Therefore, it is difficult to manufacture the MPD which has the same energy band gap as the wave length of the light rays released from the LD, and therefore, its optical detecting capability has been very low due to the non-linear optical receiving property. Therefore, research has been focused on how to form the LD and the MPD on the same chip.
FIG. 1 is a sectional view of a conventional compound semiconductor device in which an LD and an MPD are formed on the same chip. The compound semiconductor device consists of the LD formed region L and the MPD formed region M, and the LD and the MPD have the same constitution. The LD is constituted such that a p-type AlxGa .sub.1 -yAs clad layer 3, a p-type AlyGa .sub.1 -yAs activated layer 5, an n-type AlxGa .sub.1 -xAs clad layer 7, and an n-type GaAs cap layer 9 are formed on the region L of a p-type GaAs semiconductor substrate 1. The activated layer 5 is of p-type, but it can be formed of n-type, while the composition ration 1.gtoreq.x&gt;y.gtoreq.0 has to be satisfied in order for the activated layer 5 to have a refraction index larger than that of the clad layers 3,7. Further, and n-type electrode 11 is formed on cap layer 9, and a p-type electrode 13 is formed at the bottom surface of semiconductor substrate 1.
Further, the MPD having the same constitution as that of the LD is formed in the region M of the semiconductor substrate 1, and therefore, the p-type electrode 13 serves as a common electrode. The MPD receives the light rays released from the light emitting face of the LD, and, when a reverse bias is applied, the released light rays separate the carriers into electrons and holes at the activated layers 5 of the MPD as a result of which a current flows between the n-type electrode 11 and the p type electrode 13.
The amount of the above current is proportionate to the intensity of the light rays released from the light emitting face of one side of the LD, and therefore, the intensity of the light rays is controlled through an external circuit in accordance with the amount of the current. In the above, the activated layers 5 of the LD and MPD have the same energy band gaps and are also in phase as the coupling efficiency is very high. Further, if the light rays released from the LD impinge on the LD after being reflected from the light receiving face of the MPD, then the SN ratio (signal-to-noise ratio) of the LD is aggravated, and therefore, the light receiving face of the MPD is inclined relatively to the substrate 1. Thus, the semiconductor substrate 1 is etched vertically in order to form the light emitting faces of the LD, and the MPD is etched again in such a manner that the light receiving face of the MPD has a certain inclination angle relative to the light emitting face of the LD.
The above described compound semiconductor device is used as a transmitting device in communication systems, but it can be used also as a transmitting/receiving device by combining it with a receiving PD (to be called RPD below).
However, the above described compound semiconductor device requires two stages of etching in forming the light receiving face of the MPD, and therefore, its manufacturing process becomes complicated. Further, if the compound semiconductor device is used in a communication system, its combination with the RPD increases the power consumption and makes if difficult to achieve a high density, as well as increases the manufacturing cost.